Electrical rectifiers



Patented Nov. 20, 19 51- ELECTRICAL RECTIFIERS Vernon K. Kofron, St. Louis, Mo., assignor, by mesne assignments, to Vickers, Incorporated, a

corporation of Michigan No Drawing. Application January 6, 1947, Serial No. 720,479

12 Claims. 1

This invention relates to improvements in electrical rectifiers. More particularly, this invention relates to improvements in electrical rectifiers which have electropositive and electronegative elements and to improvements in making such rectifiers.

It is, therefore, an object of the present invention to provide an improved electrical rectifier which has an electropositive and an electronegative element and to provide an improved method of making such a rectifier.

In the manufacture of electrical rectifiers which have electropositive and electronegative elements, it is customary to form a very thin, high-electrical-resistivity layer or film on the surface of the electropositive element and to use that film in welding or fusing the electropositive and electronegative elements together. This is done by pressing an electronegative element into intimate contact with the thin, high-electricalresistivity layer or film on the electropositive element and holding the electronegative element against that layer or film while an electric current is passed through the electropositive and electronegative elements of the rectifier in the normal blocking direction. Where this is done, the current causes the high-electrical-resistivity layer or film to break down and permit the electropositive and electronegative elements to be fused or welded into a single element with unsymmetrical electrical characteristics. Such an element can be, and is, used to rectify alternating current, thus providing a unidirectional flow of direct current. Rectifiers of this type are generally known as dry rec'tifiers and they are quite satisfactory in operation.

In many instances, a thin, high-electricalresistivity layer or film is formed by dipping the electropositive element into an aqueous solution that contains metallic salts such as the chlorides of copper or manganese. Where the electropositive element has initially been carefully cleaned of surface oxides, as by abrading, polishing or etching, the insertion of the electropositive element into the solution of the metallic salts acts to form a thin, high-electrical-resistivity layer or film on the surface of the electropositive element. This method is workable and has been used rather extensively, but it is not as satisfactory as it could be, because this method requires careful abrading, polishing or etching of the surface of electropositive element, it requires precise control of the coating operation, and it produces a high-electrical-resistivity layer or film that is unstable in the presence of moisture.

Considerable care must be used in abrading, polishing or etching the electropositive element to free it of surface oxides, as required by previous methods of making electrical rectifiers, because the oxides must be removed to prevent their interfering with the formation of the layer or film on the electropositive element and yet the surface of the electropositive element must be kept smooth to facilitate the fusing of the electropositive and electronegative elements. As a result, a method that calls for the removal of surface oxides from an electropositive element prior to the time a high-electrical-resistivity layer or film is formed therein requires the use of highly skilled operators. In addition, such a method is so critical that uniform results cannot be obtained because of the human equation involved in carrying out the method. For these reasons, a method of making electrical rectifiers that calls for the removal of surface oxides from an electropositive element before the high-electrical-resistivity layer or film is formed on that element, is objectionable. The present invention obviates these objections by providing a method of forming a hlgh-electrical-resistivity layer or film on an electropositive element without necessitating the removal of surface oxides from the electropositive element. It is, therefore, an object of the present invention to provide a method of forming a hlgh-electrical-resistivity layer or film on the surface of an electropositive element without necessitating the removal of surface oxides from the electropositive element by abrading, polishing or etching the electropositive element.

In previous methods of making electrical rectifiers that called for the formation of a highelectrical-resistivity layer or film on the electropositive element by dipping that element in a solution of metallic salts, the electropositive element had to be assembled with and welded or fused to an electronegative element without delay because such layers or films are unstable when exposed to the moisture in the air. The instability of such layers or films is probably due to the fact that those layers are heterogeneous in nature and may include oxides or hydroxides of the metal of the electropositive element and may also include in its elemental state or in 'the form of its oxide or hydroxide, the metal that was in the solution in the form of a metallic salt. Where this is the case the ingredients of the layer or coating may, in the presence of the moisture in the air, tend to promote local electrolytic action with the metal of the electropositive element, and this electrolytic action asvaasa would cause a change in the thickness of the layer or film. This tendency toward local electrolytic action, and the somewhat indefinite composition of the layer or film on the electropositive element make it impossible to predict with certainty the nature, thickness and composition of the layer orfilm obtained where the electropositive element is inserted in a solution of metallic salts. This is objectionable sinc it makes it diillcult to obtain uniform production of electrical rectifiers. The present invention obviates this objection by providing an electropositive element with a high-electrical-resistivity layer or film which is of a homogeneous nature and of a known composition. It is, therefore, an object of the present invention to provide a method of forming a homogeneous layer or film of known composition on an electropositive element for use in making electrical rectifiers.

The local electrolytic action, between the layer or film on the electropositive element and the metal of which the electropositve element is made, is largely due to the fact that the elemental metal in the layer or film develops a potential with relation to the metal of which the electropositive element is made. The present invention avoids such a situation by providing a, layer or film on the electropositive element which cannot develop a potential relative to the metal of the electropositive element. It is, therefore, an object of the present invention to provide an electropositive element with a high-electricalresistivity layer or film that cannot develop a potential relative to the metal of the electropositive element.

' Where the high-electrical-resistivity layer or film on the electropositive element is formed by inserting that element in a solution of metallic salts, the subsequent step of fusing or welding the electropositive element to an electronegative element must be carried out in several stages. In some instances it is necessary to press the electropositive and electronegative elements together, pass an electrical current through the electropositive and electronegative elements in a normally blocking direction, cut ofi the current and permit the electronegative and electropositive elements to cool, thus completing one stage of the fusing or welding step and then to increase the pressure on the electropositive and electronegative elements, again pass the current in the normally blocking direction and again permit the electropositive and electronegative elements to cool, thus completing the second stage of the fusing or welding step. A third or fourth stage could be used, and those stages would be similar to the second .stage. This method produces satisfactory rectifiers, but the number of stages required in the fusing or welding step increases the cost of the method. The present invention eliminates several of these stages by providing a layer or film on an electropositive element that facilitates welding or fusing the electropositive to an electronegative element in one stage. It is, therefore, an object of the present invention to provide a layer or film on an electropositive element that makes it possible to weld that electropositive element to an electronegative element in one operation.

In the process of coating an electropositive element with a mixture of metallic salts by inserting the electropositive element in a solution of the metallic salts, the duration of the dipping operation, the temperature of the solution, and

4, the concentration of the solute are quite critical because the reaction between the electropositive element and the metallic salts is more or less self-perpetuating in the presence of moisture. As a result, it is difficult to obtain just the right thickness for the layer or film without having to maintain extremely close control over all the factors involved in the coating process. This is objectionable because it increases the degree of care and skill that must be possessed by the operator, and it limits the flexibility of production schedules. The present invention obviates this objection by providing an electropositive element with a layer or film that acts to seal off the electropositive element from the solution. thus tending to automatically stop the reaction between the solution and the electropositive element when the desired thickness of layer or film is attained. As a result, the time of immersion, the temperature of the solution, and the concentration of the solute are considerably less critical than in previous methods. It is, therefore, an object of the present invention to provide a method of coating electropositive elements for use in electrical rectifiers which includes forming a layer or coating on the surface of the electropositive element that will automatically seal off the electropositive element from the solution when the layer or film attains the desired thickness.

Layers or films that are produced on the surfaces or electropositive elements by immersing those elements in solutions of metallic salts are also objectionable in that they leave traces of foreign material between the electronegative and electropositive elements after the electropositive and electronegative elements have been welded or fused together. The foreign matter is objectionable because it keeps the electrical characteristics of the electrical rectifier unit from being uniform across the entire area of the electrical rectifier unit. The present invention obviates this objection by providing a layer or film on the electropositive element which will tend to disappear during the fusing or welding operation, either by volatilization or by combination with the electronegative element and as a result. no foreign matter remains in the rectifying area of the electrical rectifier unit. This tends to make the electrical characteristics of the electrical rectifier unit quite uniform across the entire area of the electrical rectifier unit. It is, therefore, an object of the present invention to provide a layer or film on an electropositive element which will either volatilize or combine with the electronegative element when the electropositive element is fused or welded to the electronegative element.

In previous methods of coating electropositive elements by immersing them in solutions of metallic salts, the thickness of the layer or film had to be controlled by careful regulation of the time of the immersion, the temperature of the solution and the concentration of the solute. The

need of controlling such a large number of fac- 'tion', selenium dioxide is admlxedwimwaie ,or after the pI-Ii of the ,.'solution i lution; jthereiu to h f proper lv'alue by the additionofgan acid ora base.

electropositive element, that selenium would pro- 1 -.vide a. layer of high-electrical-resistivity. The layer or film will be principally elemental selenium An electropositive element of magnesium. aluminum or other similar metal'isthen immersed j the solution andis permitted to acquire a layer or coating 'of elemental selenium. This layer or coating acts to seal off the electropositive element from the solution, thus automaticallyandquickly bringing the reactiontoa halt when alaye-r or film of the desired thickness has been formed. This layer or film will be substantially uniform and will have a high-electrical-resistivity and it can, after being rinsed and dried. be placed in contact with an electronegative element preparatory to the step of fusing or welding the electropositive element to the electronegative element. This is done by placing the selenium coating on the electropositive element in contact with the electronegative element and holding the elements together under pressure while an electrical current is passed through the electropositive and electronegative elements in a normally blocking direction. The selenium coating will resist the passage of the current, thus causing the fusing or welding together of the electropositive and electronegative elements to form an electrical rectifying unit and also causing the volatilization of the selenium or the combining of the selenium with the electronegative element. In either case, the selenium cannot remain as a foreign material that would keep the electrical characteristics of the junction between the electropositive and electronegative elements from being uniform throughout.

The selenium layer or film will be quite uniform in thickness andcomposition andwill provide uniform resistance for the coatedelectropositive element. In some instances, minute openings may occur in the layer or film of elemental selenium, and while those openings are not serious, they do constitute interruptions in-the layer or film. .Such openings, if any,'can easily be closed or covered by immersing the coated electropositive element in a solution of chromic acid or a solution of dichromate ions. The chromium ions in such solutions will pass through the openings in the selenium layer or film and will react with the metal of the electropositive element to form a chromate film that spans the opening, thus forming a completely uninterrupted layer or film for the electropositive element.

The process of the present invention can be carried out with solutions of varying strength and composition, and it can be carried out with different electropositive elements. However, the process can be carried out quite well where the electropositive element is of magnesium and where the solution consists of about four and one-half (4 /2) gallons of water, about nine and one-half (9 /2) pounds of anhydrous sodium selenite (NazSe03) and about one thousand to fifteen hundred (1000.-l500) cubic centimeters of concentrated hydrochloric acid. Such a solution will have a pH between two and twelve (2-12) and the solution will form a high-'electrical-resistivity layer or film of elemental selenium on the magnesium even if the magnesium has surface oxides thereon. The selenium in the layer or film will be the red or amorphous form of elemental selenium and it will act as a nonmetal. Even if the gray or crystalline selenium were formed when the layer or film is deposited on the jrbut, of course, there may be small amounts of 'selei'lite' and other materials and compounds in the layer or mm.

similar layer or mm can be obtained on a magnesium electropositive element by immersing .the element in a solution of about four (4) gallons of watenteven (7) pounds of selenium dioxide (S602) and sufilcient sodium carbonate .(NmCOa) to make the pHof the solution between two and twelve (2-12). This solution will provide a high-electrical-resistivity layer of ele-' mental selenium on the magnesium electropositive element.

Each of the above solutions will be capable of coating several hundred magnesium electropositive elements. When the solutions have been used to coat a large number of electropositive elements, a measurement should be made of the specific gravity of the solution; measurement will indicate how much material should be added to prepare the solution for additional coating operations.

The thickness of any layer will be determined by the pH of the solution. Where the pH of the solution is in the range of two to three (2-3), very heavy layers or films of selenium will be obtained on the electropositive element, and where the pH isfrom ten to twelve (10-12), very light layers or films of selenium will be obtained. A pH of from six to nine (6-9) provides a thickness of layer or film that is satisfactory for most rectifier purposes. It will be understood that the desired thickness of the film or layer will be determined in each case to fit the various mechanical and electrical characteristics of the finished rectifier and it is also to be understood that adjustment of thepH of the selenite solution enables the manufacturer to select layers or films of the desired thickness. Once the selenium film has been formed on the electropositive element, and with the examples set forth herein, it requires less than a minute for the film to form. The coated electropositive element is then removed from the solution and dried. Thereafter, the selenium coating on the rear surface of the electropositive element is brushed or scraped off and while this is a brushing or abrading operation, it need not be performed with the care that must be exercised in removing surface oxides. This brushing or scraping operation need only get the selenium oil of the back of the electropositive element for contact purposes and need not preserve a uniform surface of the type needed for fusion or welding with an electronegative element.

After the layer or film of selenium has been formed on the-electropositive element, the elec- Ill tropositlve element can be used as is or it can be immersed in a solution that contains chromium ions; The concentration of chromium ions is not critical and need not be very high since concentrations of from one-fourth to twenty-five grams of chromium compounds per liter of water are quite satisfactory. The chromium ions will coact with any exposed portions of the electropositive element to form a chromate film that will sealofl' those exposed portions from the solution. Thus the treatment with the chromium-containing solution will make the selenium layer or film continuous and uninterrupted.

' After the selenium-coated electropositive element has been removed from the chromic acid or dichromate solution, or has been removed from layer or film on ,also provides that layer or film the selenite solution as a finished product, it need only be rinsed and dried. Where desired, the electropositive element can be placed in alcohol so that the alcohol can displace the water, thus effecting a quicker drying of the electropositive element. Ordinary denatured alcohol can be used, and that alcohol should be discarded when the water content reaches the point where effective drying cannot be obtained. This alcohol dip is not an essential part of the process and is merely used to speed up the drying of the coated electropositive element. The process provided by the present invention not only provides a superior the electropositive element but it quickly and without necessitating extremely fine control of the various operations. As a matter of fact, very good rectifiers have been made from electropositive elements that were immersed in the selenite solution for approximately one minute, were immersed in the chromic acid solution for approximately ten seconds, were rinsed with water for approximately fifteen seconds, and were rinsed in alcohol for approximately fifteen seconds.

Once the selenium layer or film has been formed on the electropositive element, the electropositive element can be used immediately or can be stored for future use. There is no danger that air, and the moisture therein, will act upon the selenium and cause a change in the nature or thickness of the layer or film. This is due to the fact that the selenium will act as a nonmetal and will not establish a potential relative to the metal of the electropositive element; thus there is no possibility of having local electrolytic action between the selenium and the electropositive element. In addition, the layer or film of selenium, which acted to seal off the metal of the electropositive element from the selenite solution, will act to seal off the metal of the electropositive element from the air.

When the selenium-coated electropositive element is to be fused or welded to an electronegative element, the selenium coating-of the electropositive element is placed in contact with the electronegative element, pressure is exerted to hold the electropositive and electronegative elements in intimate engagement, and an electrical current is passed through the electropositive and electronegative elements in a normally blocking direction. Where the electronegative element is a copper sulfide disc, the electropositive element can be fused or welded to the electronegative element in one operation of from ten to twelve minutes. similar results are obtained where the electronegative element is a copper selenide or copper telluride disc.

Various selenites can be used in making the solution into which the electropositive element is to be immersed, and as a matter of fact, almost any soluble inorganic selenite can be used. In addition, soluble organic selenites, as, for example, tetraethanolammonium selenite can be used to form a layer or film of selenium on the electropositive element. The principal inorganic selenites will be the selenites of the alkaline and alkaline earth metals and those selenites work very satisfactorily.

One of the very desirable features of the present invention is its great flexibility. For example, the present invention can be carried out with selenite solutions with concentrations of from ten to one hundred (10-100) grams of selenite in each liter of water, and it can be carried out with concentrations of from one-fourth to twenty-five (%-25) grams of chromic acid or dichromate ions for each liter of water. Moreover. the strength of the selenite solution can be determined and controlled by measurement and control of the specific gravity of the solution. In general, selenite solutions with specific gravities in the range of one and twenty-five one-thousandths to one and thirty one-thousandths (1025-1030) are quite satisfactory.

Whereas, the foregoing description has described one specific process of the present invention, it should be obvious to those skilled in the art that various changes may be made in the process of the invention without a change in the scope of the invention.

What I claim is:

1. An electropositive element that can be assembled with an electronegative element to form an electrical rectifier, and that comprises a magnesium element and a uniform, high-electricalresistivity coating of elemental selenium thereon,

said selenium coating having any openings therein filled with magnesium chromate.

2. The method of making rectifierswith electropositive and electronegative elements that 3. The method of making rectifiers with electropositive and electronegative elements that comprises making an electropositive element, which can be assembled with an electronegative element to form an electrical rectifier, by inserting an electropositive element in a selenite solution and permitting the metal of the electropositive element to attain a coating of elemental selenium, placing said film or layer in contact with an electronegative element, and passing current through said electropositive and electronegative elements.

4. The method of making an electropositiveelectronegative rectifier that comprises forming on the surface of the electropositive element a high-electrical-resistivity film in elemental form that will be absorbed by the electronegative element during the welding process, placing the coated surface of the electropositive element in contact with the surface of the electronegative element, and then passing a. current through the electropositive and electronegative elements to weld the two together and to cause the material of the film to be absorbed by the electronegative element.

5. The method of forming an electropositive element with a uniform, high-electrical-resistivity coating thereon, which can be assembled with an electronegative element to form an electrical rectifier, that comprises dipping the electropositive element in a solution of selenium ions to form a coating of elemental selenium on the electropositive element, and thereafter dipping the coated electropositive element in a solution of chromium ions so that the chromium ions can coact with the metal of the electropositive ele- 9 ment to seal openings in the selenium layer on the electropositive element.

6. The method of forming an electropositive element with a nonporous coating thereon, which can be assembled with an electronegative element to form an electrical rectifier, that comprises immersing a coated electropositive element in a solution of chromium ions, whereby the chromium ions of the solution can pass through openings in the coating of the coated electropositive element and contact the metal of that element to form chromates that will fill and span those openings.

7. The method of making rectifiers with electropositive and electronegative elements that comprises making an electropositive element, which'can be assembled with an electronegative element to form an electrical rectifier,by immersing a magnesium electropositive element in a solution that contains selenium ions and has a pH in the range of two to twelve (2-12) until a uniform, high-electrical-resistivity layer of elemental selenium is formed on said element, placing said film or layer in contact with an electronegative element, and passing current through said electropositive and electronegative elements.

8. The method of making rectifiers with electropositive and electronegative elements that comprises making a coated electropositive element, which can be assembled with an electronegative element to form an electrical rectifier, by adding a soluble selenite to water, adding an acid to reduce the alkalinity of the solution, and immersing the electropositive element in the solution until a uniform coating of elementalselenium is formed thereon, placing said film or layer in contact with an electronegative element, and passing current through said electropositive and electronegative elements.

9. The method of making rectifiers with electropositive and electronegative elements that comprises making a coated electropositive element, which can be assembled with an electronegative element to form an electrical rectifier, by adding selenium dioxide to water, adding a base to reduce the acidity of the solution, and immersing the electropositive element in the solution until a uniform coating of elemental selenium is formed thereon, placing said film or layer in contact with an electronegative element, and passing current through said electropositive and electronegative elements.

10. The method of making an electrical rectifier that comprises inserting a magnesium electropositive element in a solution to form an elemental selenium coating thereon, placing the coated surface of the magnesium electropositive element in contact with the surface of an electronegative element, and passing a current in the normally blockingdirection through said electronegative and magnesium element.

11. In the manufacture of electropositive-electronegative electrical rectifiers, an electropositive element of magnesium, a uniform high-electricalresistivity coating of elemental selenium on one surface thereof, and an electronegative element in direct and intimate engagement with said coating of elemental selenium.

12. In the manufacture of electropositive-electronegative electrical rectifiers, a magnesium electropositive element, an electronegative element having one surface thereof adjacent a surface of said magnesium electropositive element, and a layer of elemental selenium on and intimately adherent to said surface of said magnesium electropositive element, said layer of elemental selenium bearing directly against the said one surface of said electronegative element and constituting a uniform high-electrical-resistivity coating. I

VERNON K. KOFRON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,826,955 Ruben Oct. 13, 1931 1,858,445 Hambuechen May 17, 1932 1,866,351 Hollnagel et al. July 5, 1932 1,961,030 Bengough May 29, 1934 1,961,825 Harty June 5, 1934 2,175,873 Brunke et al. Oct. 10, 1939 2,189,576 Brunke Feb, 6, 1940 2,203,670 Buzzard June 11, 1940 2,266,380 Neely Dec. 16, 1941 2,364,436 Frisch Dec. 5, 1944 2,378,438 Saslaw June 19, 1945 2,408,116 Von Hippel Sept. 24, 1946 2,426,173 Bloom Aug. 26, 1947 2,453,763 Smith Nov. 16, 1948 Hewlett July 12. 1949 

